In a radio frequency (RF) transmitter, an IQ modulator is used for modulating source information from in-phase (I) and quadrature (Q) signal components. Baseband I and Q signals are commonly used for modulation, as these signals or baseband waveforms are easier to manipulate as compared to a higher frequency or bandpass waveform signals. Translation from baseband signals to an RF bandpass signal is achieved by mixing the baseband IQ signal components with a reference signal (generated by a local oscillator (LO)) at a carrier frequency. The nonlinear mixing and summation of the two baseband IQ signals results in a modulated RF signal that may then be amplified and transmitted. Direct conversion transmitters use an LO synchronized to the exact frequency of the carrier in order to directly translate the baseband signals to RF frequencies.
Unfortunately, IQ modulators are susceptible to carrier leakage such that a significant amount of energy at the carrier frequency is present at the output of the transmitter. Carrier leakage can originate from DC offsets generated in the baseband source, or can be injected by baseband signal processing elements or imbalances within the mixers, any of which potentially impose unacceptable amounts of energy at the carrier frequency. Accordingly, there is a need for measuring and reducing the amount of carrier leakage.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.